Direct Observation of Ammonia Storage in UiO-66 Incorporating Cu(II) Binding Sites

The presence of active sites in metal–organic framework (MOF) materials can control and affect their performance significantly in adsorption and catalysis. However, revealing the interactions between the substrate and active sites in MOFs at atomic precision remains a challenging task. Here, we report the direct observation of binding of NH3 in a series of UiO-66 materials containing atomically dispersed defects and open Cu(I) and Cu(II) sites. While all MOFs in this series exhibit similar surface areas (1111–1135 m2 g–1), decoration of the −OH site in UiO-66-defect with Cu(II) results in a 43% enhancement of the isothermal uptake of NH3 at 273 K and 1.0 bar from 11.8 in UiO-66-defect to 16.9 mmol g–1 in UiO-66-CuII. A 100% enhancement of dynamic adsorption of NH3 at a concentration level of 630 ppm from 2.07 mmol g–1 in UiO-66-defect to 4.15 mmol g–1 in UiO-66-CuII at 298 K is observed. In situ neutron powder diffraction, inelastic neutron scattering, and electron paramagnetic resonance, solid-state nuclear magnetic resonance, and infrared spectroscopies, coupled with modeling reveal that the enhanced NH3 uptake in UiO-66-CuII originates from a {Cu(II)···NH3} interaction, with a reversible change in geometry at Cu(II) from near-linear to trigonal coordination. This work represents the first example of structural elucidation of NH3 binding in MOFs containing open metal sites and will inform the design of new efficient MOF sorbents by targeted control of active sites for NH3 capture and storage.


■ INTRODUCTION
Ammonia (NH 3 ) is a major feedstock in the agricultural and chemical industries, 1 but due to its toxic and corrosive nature, storage and transport of NH 3 in large quantities is challenging. 2 It is therefore of great interest to develop efficient sorbent materials that show significant chemical and physical stability and high adsorption capacity for NH 3 . Conventional sorbents, including zeolites, 3 activated carbons, 4 mesoporous silica, 5 and resins, 6 have been studied for NH 3 adsorption, but they show limited capacities and often undergo irreversible structural degradation upon desorption. In addition, fine-tuning and directed chemical manipulation of active sites in these materials at the atomic level can be problematic due to the lack of direct structural insights and limited structural diversity.
Porous metal−organic framework (MOF) materials adopt well-defined structures, are designable, and can show exceptional structural diversity, enabling the control of active sites at atomic precision. 7 (4,5-d′)bistriazole], 19 HKUST-1, 20 and UiO-67, 21 have been reported for NH 3 adsorption. However, it remains highly challenging to identify the precise role of these active sites in binding NH 3 molecules, not least because of the relative invisibility of protons in NH 3 by X-ray diffraction and the complex and rapid host−guest dynamics involved in NH 3 binding. Revealing such insights will enable targeted control of active sites and thus deliver efficient NH 3 stores by design. This will further inform the development of next-generation catalysts for the cracking of NH 3 for portable applications relating to the hydrogen economy.
Here, we report the study of binding domains and dynamics of NH 3 within UiO-66-defect (UiO-66 with a missing carboxylate ligand), UiO-66-Cu I , and UiO-66-Cu II based upon the direct observation of the location of atomically dispersed active sites and their interactions with NH 3 molecules. The robustness of the framework in UiO-66 makes it an ideal platform for the study of NH 3 adsorption, and the incorporation of open Cu(II) sites can provide further strong binding and activation sites. Compared with UiO-66defect, UiO-66-Cu II shows significant enhancement of static (11.8 and 16.9 mmol g −1 , respectively, at 273 K and 1.0 bar) and dynamic (2.07 and 4.15 mmol g −1 , respectively, at 298 K and at 630 ppm concentrations) adsorption of NH 3 , thus serving as a top-performing NH 3 sorbent. In situ neutron powder diffraction (NPD), inelastic neutron scattering (INS), coupled with density functional theory (DFT) modeling, electron paramagnetic resonance (EPR), solid-state nuclear magnetic resonance (ssNMR), infrared (IR), and ultraviolet− visible (UV−vis) absorption spectroscopies reveal the presence of reversible {Cu(II)···NH 3 } interactions that underpin the observed high and reversible NH 3 uptake. ■ EXPERIMENTAL SECTION NH 3 Adsorption Isotherms and Cycling Experiment. The synthesis and activation of MOF materials have been reported in our previous study 22 and are described in detail in the Supporting Information. Static adsorption isotherms (0−1.0 bar) for NH 3 were measured on IGA (intelligent gravimetric analyzer, Hiden Isochema, Warrington, U.K.). Desolvated samples of UiO-66-defect, UiO-66-Cu I , and UiO-66-Cu II were generated in situ under dynamic vacuum (1 × 10 −8 mbar) at 393 K for 24 h. NH 3 (research-grade) was purchased from BOC and used as received. For cycling experiments, the pressure of NH 3 was increased from vacuum (1 × 10 −8 mbar) to 150 mbar and the uptake was recorded. The pressure was then reduced to regenerate the sample with no assisted heating. This cycling process was repeated for 15 cycles.
Neutron Powder Diffraction (NPD). The binding positions of ND 3 within UiO-66-defect and UiO-66-Cu II were determined by NPD experiments at WISH, a long-wavelength powder and singlecrystal neutron diffractometer at the ISIS Facility at the Rutherford Appleton Laboratory (U.K.). 23 Prior to NPD measurements, the sample was activated by heating at 393 K under dynamic vacuum for 16 h, and the desolvated samples were then transferred into cylindrical vanadium sample cells with an indium seal. The samples were further degassed at 373 K under dynamic vacuum to remove the remaining trace guest water molecules. Dosing of ND 3 was carried out volumetrically at room temperature to ensure that ND 3 was present in the gas phase when not adsorbed and to ensure sufficient mobility of ND 3 inside the MOF framework. The temperature during data collection was controlled using a helium (He) cryostat (7 ± 0.2 K). The quality of the Rietveld refinements has been assured with low goodness-of-fit (Gof) factors, low weighted profile factors (R wp ), and well-fitted patterns with reasonable isotropic displacement factors.
Inelastic Neutron Scattering (INS). INS experiments were performed at TOSCA neutron spectrometer at the ISIS Facility at the Rutherford Appleton Laboratory (U.K.). 24 Desolvated UiO-66-defect and UiO-66-Cu II materials were loaded into cylindrical vanadium sample cells with an indium seal and degassed at 373 K under dynamic vacuum to remove the remaining trace guest water molecules. The temperature during data collection was controlled using a He cryostat (7 ± 0.2 K). The loading of NH 3 was performed volumetrically at room temperature, and background spectra of bare MOF samples were subtracted to obtain the difference spectra.
Solid-State Nuclear Magnetic Resonance (ssNMR) Spectroscopy. Magic angle spinning (MAS) NMR spectra were recorded using a Bruker 9.4 T (400 MHz 1 H Larmor frequency) AVANCE III spectrometer equipped with a 4 mm HFX MAS probe. Samples were desolvated and packed into 4 mm o.d. zirconia rotors under inert conditions and sealed with a Kel-F rotor cap. Experiments were carried out at ambient temperature using a MAS frequency of 12 kHz. 1 H-pulses of 100 kHz and 13 C-pulses of 50 kHz were used, and 13 C spin-locking at ∼50 kHz was applied for 2 ms, with corresponding ramped (70−100%) 1 H spin-locking at ∼73 kHz for CP experiments and with 100 kHz of SPINAL-64 25 heteronuclear 1 H decoupling throughout signal acquisition. Then, 640−8192 transients were coadded for the CPMAS NMR spectra, depending on the sample. 1 H Hahn echo spectra used an inter-pulse delay of one rotor period, giving a total echo time of 0.167 ms. For the two-dimensional (2D) 1 H− 13 C FSLG-HETCOR 26 dipolar correlation experiment, 2304 Journal of the American Chemical Society pubs.acs.org/JACS Article transients were acquired for each of 32 complex t 1 increments and a CP contact time of 0.5 ms was employed. Electron Paramagnetic Resonance (EPR) Spectroscopy. CW EPR spectra were measured with a Bruker EMX 300 EPR spectrometer equipped with a high sensitivity X-band (ca. 9.4 GHz) resonator and a liquid He cryostat. The spectra were recorded at a microwave power of 0.0022−2.2 mW, modulation frequency 100 kHz, and modulation amplitude 5 G. Field corrections were applied by measuring relevant EPR standards (Bruker Strong Pitch and DPPH). Pulsed EPR measurements were performed at X-band (ca. 9.7 GHz) on a Bruker ElexSys E580 spectrometer. The microwave frequency was measured with a built-in digital counter, and the magnetic field was calibrated using a Bruker strong pitch reference sample.
■ RESULTS AND DISCUSSION NH 3 Adsorption Analysis. Desolvated UiO-66-defect, UiO-66-Cu I , and UiO-66-Cu II display BET surface areas of 1135, 1111, and 1124 m 2 g −1 , respectively (Table S1). Thus, the variation of active sites in the pore interior has little impact on the porosity of resultant UiO-66 materials. X-ray absorption fine structure (XAFS) spectroscopy of UiO-66-Cu II ( Figure  S2) shows a lower intensity for the features at a long distance (∼2.5 Å) in the Fourier transform of the k 2 -weighted data compared with that for CuO as a reference material. This strongly suggests that the Cu(II) sites in UiO-66-Cu II are atomically dispersed, 22,27 in full agreement with the EPR spectroscopic results, which confirmed the absence of aggregated (long-range magnetically coupled) or binuclear (S = 1) Cu(II) species in UiO-66-Cu II . 22 At 273 K and 1.0 bar, UiO-66-defect, UiO-66-Cu I , and UiO-66-Cu II exhibit NH 3 uptakes of 11.8, 12.6, and 16.9 mmol g −1 (Figure 1a−c), respectively, comparable with state-of-the-art materials (Table  S7). The isotherms display apparent hysteresis loops ( Figure  S3), indicating the presence of strong host−guest interactions at the binding sites within the two types of cages of the framework (tetrahedral and octahedral cages with diameters of 7 and 9 Å, respectively). UiO-66-defect, UiO-66-Cu I , and UiO-66-Cu II show high packing density of NH 3 of 0.52, 0.55, and 0.74 g cm −3 , respectively, demonstrating efficient volumetric storage of NH 3 (Table S3). It is worth noting that the packing density of UiO-66-Cu II is comparable to that of liquid NH 3 (0.68 g cm −3 ) at 240 K. All three MOFs show high stability toward pressure-swing NH 3 adsorption with retention of the structure, porosity, and NH 3 uptakes over at least 15 cycles (Figures 1e, S1, S4, and S6). This is in direct contrast to reported MOFs incorporating four-or five-coordinated open Cu(II) sites that lead to irreversible sorption of NH 3 and structural degradation upon desorption. 19,28−30 Upon desorption under pressure-swing conditions, higher residues of NH 3 in UiO-66-Cu I and UiO-66-Cu II (49−67%) were observed compared with UiO-66-defect (27−30%), attributed to the interactions between NH 3 molecules and Cu sites (vide infra). The residual NH 3 in all three systems can be fully released via heating, reflecting a relatively strong binding of NH 3 in these MOFs. The excellent ability of UiO-66-defect, UiO-66-Cu I , and UiO-66-Cu II to capture NH 3 at low concentrations (630 ppm) has been confirmed by dynamic breakthrough experiments at 298 K, where the dynamic NH 3 uptakes were calculated to be 2.07, 3.07, and 4.15 mmol g −1 , respectively ( Figure 1d). The introduction of Cu(II) sites leads to 100% enhancement of the dynamic NH 3 adsorption capacity at low concentrations, which is highly desirable for the capture of NH 3 as a pollutant and/or at low concentrations. With increasing loading of NH 3 , the isosteric heat of adsorption (Q st ) increases, and the adsorption entropy (ΔS) decreases for all three MOFs, indicating the presence of strong intermolecular interactions and ordering of adsorbed NH 3 molecules in the pore ( Figure S5). As expected, UiO-66-Cu II shows higher Q st than UiO-66-defect and UiO-66-Cu I (up to 55, 40, and 35 kJ mol −1 , respectively).
Determination of the Binding Sites for Adsorbed ND 3 . Rietveld refinements of the in situ NPD data collected at      Figure S17). 36 The presence of strong binding of NH 3 to Cu(I) and Cu(II) sites was also confirmed by temperatureprogrammed desorption of NH 3 (NH 3 −TPD) ( Figure S18) and 1 H ssNMR spectroscopy (Figure 5a). The additional TPD peaks at higher temperatures (150−300°C, Figure S18b) for UiO-66-Cu I and UiO-66-Cu II , which are not observed in UiO-66-defect, indicate stronger binding of NH 3 at these Cu sites. In addition, the desorption peaks for UiO-66-Cu II appear at higher temperatures compared with those of UiO-66-Cu I , suggesting a stronger {Cu(II)···NH 3 } interaction than {Cu-(I)···NH 3 }, consistent with the adsorption and breakthrough results. These conclusions are supported further by the corresponding 1 H magic angle spinning (MAS) NMR spectra of the NH 3 -loaded materials (Figure 5a). For UiO-66-defect, a large narrow signal from NH 3 is observed (FWHM ∼ 650 Hz, centered at δ{ 1 H} = 2.8 ppm), suggesting rapid relative motion of NH 3 in the pores. For UiO-66-Cu II and UiO-66-Cu I , this large peak is absent but a broad signal (FWHM ∼ 2 kHz, centered at δ{ 1 H} = 3.7 ppm) is present that stems from poreconfined NH 3 ( Figure S19). Furthermore, for UiO-66-Cu I and UiO-66-Cu II , very broad signals are observed at negative chemical shifts (FWHM ∼ 7 kHz, centered at δ{ 1 H} = −7.6 ppm for UiO-66-Cu II and δ{ 1 H} = −15.0 ppm for UiO-66-Cu I ) corresponding to metal-bound NH 3 , 37 again consistent with the infrared and UV−vis spectroscopic studies. Onedimensional (1D) 13 C and 2D 1 H− 13 C dipolar correlation MAS NMR spectra ( Figure S19) also indicate a hydrogen- EPR spectra of UiO-66-Cu II recorded at 40 K before adsorption of NH 3 (red, pre-activated solvated form), after adsorption of NH 3 (blue), desorption of NH 3 (green), and after exposure to the air for more than 24 h (black). (c) X-band (9.4 GHz) EPR spectra of UiO-66-Cu II at 6 K after NH 3 loading. Blue: CW spectra; black: echo-detected spectra recorded with π/2 = 16 ns and τ = 150 ns; and light green: derivative of echodetected spectra recorded with π/2 = 16 ns and τ = 150 ns. (d) Relative quantities of the broad (square, orange and deep green) and isolated Cu(II) (circle, light orange and lime green) EPR signals upon degassing NH 3 @UiO-66-Cu II (orange) and NH 3 @UiO-66-Cu I (green) with heating under dynamic vacuum (see SI for details). (e) In situ infrared spectra of UiO-66-Cu II upon adsorption and desorption of NH 3 .
Journal of the American Chemical Society pubs.acs.org/JACS Article bonding interaction between NH 3 molecules and the carboxylate moieties from the organic linkers of the MOFs. The strong interaction between NH 3 and Cu(II) site was further elucidated by EPR spectroscopy. UiO-66-Cu II in its hydrated form has an X-band continuous wave (CW) EPR spectrum (Figure 5b), with axial (within the resolution of the experiment) spin Hamiltonian parameters [g x,y = 2.074, g z = 2.320, A z(Cu) = 480 MHz], typical of isolated Cu(II) ions with a d x 2 −y 2 or d xy ground state and consistent with water coordinated in the xy plane; the latter is confirmed by HYSCORE measurements. 22 Dehydration of the sample leads to loss of the coordinated water ( Figures S20, S21, and S23) and significant intensity loss in the CW EPR spectrum. 22 This phenomenon has been observed in several Cu(II)-doped zeolites [without reduction to Cu(I)], attributed to unusual low-coordinate geometries that can lead to near-degenerate ground states. 38 This is also consistent with the NPD model that suggests a pseudo-linear geometry at the Cu(II) site ( Figure 2c) and with the observation that the spectra (CW EPR, HYSCORE) of the solvated system are not restored by exposure to dry O 2 but are restored by exposure to air via the uptake of moisture.
Adsorption of NH 3 in an activated sample of UiO-66-Cu II led to the recovery of the intensity of the signal within the CW EPR spectrum, indicating that the adsorbed NH 3 interacts with the Cu(II) sites. Two components are observed: (i) an isolated Cu(II) signal with modified parameters [g x,y = 2.07, g z = 2.27, A z(Cu) = 530 MHz] and (ii) an unresolved, broad signal at g ≈ 2.115 (Figures 5b and S20, and Table S5). Only the former is observed in echo-detected EPR spectroscopy (Figures 5c and  S22), demonstrating their different origin and that the species giving rise to the broad signal relaxes quickly. The observed decrease in g z and increase in A z(Cu) (compared to the hydrated form) of the anisotropic component are consistent with a mixed O/N-donor set, and similar changes have been observed in NH 3 binding in Cu-doped zeolites. 39 The origin of the broad signal is less clear: the lack of resolution and rapid relaxation may indicate exchange interactions between the Cu(II) ions. The nearest possible intra-and inter-node Cu··· Cu distances are 4.4 and 5.8 Å, respectively, and an interaction would only need to be a few hundred MHz to affect the CW EPR response significantly. This could be mediated by the hydrogen-bonding network of adsorbed NH 3 molecules within the tetrahedral cages. Since the defects will be distributed statistically, both coupled and uncoupled spectra could be observed. An alternative explanation for the broad signal would be a fluxional process, which averages the EPR response. However, the spectra are unchanged on cooling to 4 K, which should freeze out any such process. Similar broad, nearisotropic CW EPR signals have been observed on NH 3 loading on the Cu(II)-MOF, HKUST-1, tentatively attributed to spinexchange phenomena. 29 In contrast to the HKUST-1 study, where the changes in NH 3 adsorption were irreversible, 29 the CW EPR and HYSCORE spectra of UiO-66-Cu II can be readily regenerated by degassing and exposure to air, confirming excellent stability and reversibility of this system (Figures 5b, S21, and S23). At 10 −7 mbar at different temperatures, the broad signal is lost first (Figure S20), consistent with preferential loss of NH 3 molecules at Site II/ III, which disrupts the hydrogen-bonding network that facilitates the Cu···Cu interaction.
Similar EPR spectra and behavior are found for UiO-66-Cu I , which indicates the presence of a minor amount of Cu(II) ions along with Cu(I) sites after the reduction. To compare the strength of Cu···NH 3 binding in UiO-66-Cu II and UiO-66-Cu I materials, the desorption and regeneration processes after NH 3 adsorption were compared (Figures 5d and S21). The broad isotropic signal is lost more quickly for UiO-66-Cu I , demonstrating stronger binding in the UiO-66-Cu II system, consistent with the TPD and ssNMR analyses and the isothermal adsorption and breakthrough results.

■ CONCLUSIONS
In summary, robust UiO-66 materials incorporating atomically dispersed defects and open Cu(I) and Cu(II) sites show high and reversible NH 3 adsorption capacities. While the decoration of defects with open Cu(I) and Cu(II) sites exhibits little change to the BET surface area (1111−1135 m 2 g −1 ), the latter Cu(II) system shows 43 and 100% enhancements in the static and dynamic adsorption of NH 3 , respectively, compared with UiO-66-defect. This places UiO-66-Cu II as one of the state-ofthe-art NH 3 sorbents. The host−guest interactions between the frameworks and adsorbed NH 3 molecules have been investigated comprehensively at the molecular level. In situ NPD, ssNMR, EPR, IR, UV−vis, and INS/DFT studies have established the binding interactions between NH 3 and defect sites and the critical role of low-coordinate Cu(II) sites in stabilizing NH 3 molecules has been determined unambiguously. This is distinct from four and five-coordinated Cu(II) sites that lead to irreversible structural degradation upon desorption of NH 3 . By combining NH 3 −TPD, in situ ssNMR, IR, UV−vis, and EPR experiments, the host−guest interactions have been revealed, and this is accompanied by a reversible change of the unique, near-linear coordination geometry of Cu(II) sites as a function of NH 3 binding. This is the structural origin of the observed reversible NH 3 adsorption in this system involving open metal sites. These findings showcase the designed tuning of active sites in MOFs that can result in topperforming NH 3 adsorption without altering the porosity of a given material. We anticipate that this study will provide key insights into the design and preparation of new efficient sorbents for NH 3 via the full control of active sites with atomic precision.